1. Field of the Invention
This invention relates to a pneumatic tire, and more particularly to a pneumatic radial tire for use in a heavy vehicles such as truck, bus and the like having an aspect ratio of not more than 70 and having excellent bead portion durability.
2. Description of Related Art
In pneumatic radial tires running under load, there is caused a so-called bead portion fall-down phenomenon where a pair of sidewall portions are largely flexed at a position corresponding to a ground contact region but also a bead portion located outside a flange of a rim in a radial direction of the tire falls down toward the outside of the tire. Based on such a phenomenon, a large compression strain is applied to the inside of the bead portion, occasionally an end part of a turnup portion of a carcass existing in the sidewall portion near the bead portion.
Further, the movement of a tire member is caused along substantially a circumference of the tire in a zone ranging from the bead portion to the sidewall portion, which corresponds to positions of a leading part and a trailing part in the ground contact region of the tire tread portion. Hence, a shearing strain along the circumference of the tire is also applied to the turnup portion of the carcass based on such a movement. As a result, the shearing strain and the compression strain are repeatedly input to the end part of the carcass turnup portion during the running of the tire under loading, rubber fatigue at the end part of the carcass turnup portion is promoted and finally cracks are apt to be caused in the rubber along such an end part. Once the rubber cracking is caused, it progresses up to the occurrence of separation failure as the running distance of the tire becomes long.
Such problems in the bead portion of pneumatic radial tires for passenger cars running under a relatively low load are scarce, but is easily caused in large-size pneumatic radial tires for heavy vehicles such as truck, bus and the like running under a heavy load. This is particularly true in low-section profile tires (tubeless tires) recently increasing their use frequency and having an aspect ratio of not more than 70. Therefore, this trouble is a serious problem in such tire users and the solution thereof is strongly demanded. As to the large-size radial tire, there have been taken the following countermeasures for improving the separation failure in the carcass turnup portion.
In the first countermeasure, as shown by a diagrammatically section view of a part of the tire ranging from a bead portion 1 to a sidewall portion 2 in FIG. 14, a bead portion reinforcing cord layer 8 is arranged so that an end of the cord layer 8 is terminated inward from an end of a turnup portion 5b of a carcass 5 wound around a bead core 4 from an inside of the tire toward an outside thereof in the radial direction and the other end is extended along an outside face of the turnup portion 5b to an inside face of a main body 5a of the carcass 5. In this case, the other end of the reinforcing cord layer 8 at the side of the main body 5a of the carcass 5 is terminated outward from the end of the turnup portion 5b in the radial direction. Also, steel cords are used in the reinforcing cord layer 8. Moreover, reference numeral 7A is a bead portion reinforcing rubber known as stiffener rubber.
In general, the second countermeasure is a widely used means. As shown by a partial section view in FIG. 15, a stiffener rubber 7A taperingly extending between the main body 5a of the carcass and the turnup portion 5b thereof outward from the bead core 4 along the main body of the carcass in the radial direction. It is constructed with a hard stiffener rubber segment 7A-1 arranged inward along the main body of the carcass in the radial direction and having a substantially a triangular shape at a section thereof and a soft stiffener rubber segment contacting with an outer face of the rubber segment 7A-1 and arranged along the inner face of the turnup portion 5b of the carcass. The soft stiffener rubber segment 7A-2 has a Shore A hardness of not more than 55.degree. and a volume ratio of the soft stiffener rubber segment 7A-2 occupied in the stiffener rubber 7A is at least 10% but is made smaller than that of the hard stiffener rubber segment 7A-1. Moreover, steel cords are applicable to the bead portion reinforcing cord layer 8.
The third countermeasure is a unique means for improving the durability of the bead portion as disclosed in JP-A-8-225005. As shown in FIG. 16, a hard stiffener rubber segment 7B-1 in a stiffener rubber 7B is arranged near the main body 5a of the carcass as a deformation input-blocking rubber member having a maximum thickness portion at least located toward the side of the tread portion from a vertical line Lv drawn from the end 5bE of the turnup portion 5b to the outer face of the main body 5a of the carcass. A soft stiffener rubber segment 7B-2 is arranged along an inner face of the turnup portion 5b. This bead portion durability improving means is based on a technical idea entirely different from strain is mitigation and strain control of the first and second countermeasures and has a dominant position over the first and second countermeasures.
As means for improving the carcass turnup portion of the large-size pneumatic radial tire, according to the above first countermeasure, tension produced at the side of the main body 5a of the carcass 5 from the bead portion 1 to the sidewall portion 2 during the running of the tire under loading is effectively utilized by the bead portion reinforcing cord layer 8 to improve tensile rigidity. Also compression rigidity is improved by arranging the reinforcing cord layer 8 against the fall-down phenomenon at the side of the turnup portion 5b. Therefore, the effect of controlling the fall-down phenomenon is developed by the improvement of both the rigidities, whereby compression strain applied to the end part of the turnup portion 5b is reduced to improve the bead portion durability.
The second countermeasure develops the effect of controlling the fall-down of the bead portion 1 by the hard rubber stiffener rubber segment 7A-1 and the effect of mitigating compression strain applied to the end part of the turnup portion 5b by the soft stiffener rubber segment 7A-2, so that it is said to be an effective means for improving the bead portion durability. Therefore, the separation failure of the bead portion 1 is improved by using the first and second countermeasures together as compared with the conventional technique.
In general, the belt produces a special torsional deformation force in both a deformation beginning zone and deformation returning zone of the belt at leading end part just before and after the entrance into a state of contacting the tread portion of the tire during the running under loading with ground and trailing end part just before and after the separation of the tread portion from the ground contacting state. Such a deformation force distorts the sidewall portion through the carcass. Such distorting is applied to the turnup portion as a large shearing strain along the circumference of the bead portion to bring about the occurrence of separation failure in the turnup portion based on such a special shearing strain. The third countermeasure advantageously controls the occurrence of such a separation failure in form of adding to the first or second countermeasure, so that it is a considerably advanced means and produces a peculiar function and effect entirely different from those of the first and second countermeasures.
Recently, the use ratio of a low-section profile large-size radial tire (particularly for use in truck and bus) having an aspect ratio of not more than 70 is increasing, and hence the more prolongation of wear life and the more improvement of belt durability in the tread rubber of a new large-size radial tire are rapidly advanced to prolong the service life of the new tire. On the other hand, it is considerably demanded to more increase the recapping number and the recapping ratio of these tires. Under the above circumstances, it is the present condition that the above three countermeasures can not provide tires having a bead portion durability capable of sufficiently following to persistent change of such a tire development.